The present invention generally relates to methods for identifying the location of movable objects. More particularly, the present invention relates to reducing capacity requirements for identifying the current location of movable objects or vehicles wherein the location is communicated over capacity-constrained wireless systems.
Typical modern systems that track the location of movable objects or vehicles utilize equipment located within the vehicle or object that incorporates a Global Positioning System (GPS) receiver. The GPS receiver captures the absolute coordinates, expressed as latitude and longitude, of the vehicle or object and conveys the absolute coordinates to a principal entity, such as a personal computer, fleet vehicle dispatch center, or rental car terminal, via a wireless communications systems.
Existing systems convey absolute coordinate information utilizing wireless communication systems that have significant capacity available, such as, but not limited to, the Advanced Mobile Phone Service (AMPS) analog system, Digital AMPS (DAMPS) known individually as Code Division Multiple Access (CDMA/IS-95) and Time Division Multiple Access (TDMA/IS-136), the Global System for Mobile communications (GSM), Enhanced Data Rates for Global Evolution (EDGE), General Packet Radio Service (GPRS), and two-way paging protocols. Such wireless systems typically possess data capacity of 8,000 bits per second or more. As such, existing systems for conveying geographic location information fail to teach methods to convey comparable location information over capacity-constrained wireless communications systems, including low capacity systems. Examples of low capacity wireless communications systems include the Cellemetry(copyright) Data Service, which has an uplink payload size of 32 bits, and some satellite data systems, such as Vistar Datacomm""s GlobalWave(trademark) system which has an uplink payload size of 88 bits.
Some existing systems reduce the message size necessary to transmit latitude and longitude values to a wireless remote (or mobile) unit by communicating only the arithmetic difference between a constant value and the desired destination (also known as a reference point) relative to the constant value. An example of a commonly used constant or predefined value is a known geographic location of a base (cell) site. Thus, all reference point values are relative to a fixed, constant location known to both ends of the communication and separately identified to each end for correlation either by token or contextual association.
Stated differently, existing systems communicate a numeric value that is the arithmetic difference of the absolute geographic coordinates of a reference point (i.e., a variable geographic location) and the absolute geographic coordinates of a reference geographic location (i.e., a constant, predefined geographic coordinate known to both ends of the wireless communication), as well the identity of the reference geographic location. An example of a commonly used reference geographic location is a wireless system""s base site identity (e.g., a radiotelephone system""s transceiver site, i.e., BASE_ID of TIA/EIA/IS-95A). The identity of the constant absolute geographic coordinate must be either physically transmitted, thus consuming additional wireless capacity, or contextually conveyed by virtue of the base site with which the remote unit communicates. In either case, the number of reference geographic locations that can be predefined and identified by a token, such as the base site identity, is finite and limited to the number of base sites associated with the host wireless system.
As the remote unit increases in distance from the reference geographic location, the arithmetic difference of the respective coordinates increases in size, along with the number of bits required to express the value of the arithmetic difference. Since the number of reference geographic locations that can be known and identified to both ends of a communication is finite, the remote unit""s distance from a known reference geographic location may become large. In such instances, the size, in bits, needed to express the arithmetic difference also grows and ultimately exceeds the payload of capacity-constrained wireless systems, such as the Cellemetry(copyright) Data Service and Vistar Datacom""s GlobalWave(trademark) system.
Existing systems fail to teach methods of reducing the message size needed to convey a relative coordinate when referring to a geographic location""s absolute coordinate, where the geographic location is dynamic and the absolute coordinates of the geographic location are not predefined. Additionally, existing systems fail to teach maintaining a reduced message size to convey a relative coordinate regardless of the distance traveled.
Accordingly, there exists a need to remove the messaging constraints of existing systems by eliminating the need to identify a predefined, constant absolute reference geographic location by token or other identification means. Furthermore, eliminating the need for remote mobile units to have prior knowledge of one or more fixed geographic reference points, either preloaded or downloaded via a wireless communications systems, will reduce the cost and complexity for remote mobile units of movable object tracking systems.
There is also a need to reduce message size while identifying an unlimited number of geographic locations without requiring fixed, constant locations known to both ends of the communication, regardless of the distance traveled. Additionally, there is a need to maintain a small message size regardless of the distance traveled.
The present invention enables efficient communication of location identification information for remote movable objects or vehicles over capacity-constrained wireless communications systems by transmitting only the truncated latitude and longitude coordinate data relative to a dynamically alterable coordinate reference anchor. In an exemplary embodiment according to this invention, the absolute latitude and longitude coordinates of the mobile object""s current geographic location are transmitted to a principal entity using one or more message transmissions, as necessary to convey 41 bits, via a low capacity wireless communications system. Subsequent current positions are conveyed with efficient, single message transmissions by encoding only the difference between the current location and that of the previous location most recently identified by absolute latitude and longitude coordinates.
According to an exemplary embodiment, a mobile object operates within an imaginary grid comprising a 4xc2x0 by 4xc2x0 geographic area. The center of the geographic area is the position of the most recently transmitted absolute longitude and latitude coordinates or full position transmission. The center and location of any grid are initially arbitrary and not predefined, and thus the potential number of grids is infinite. The full position transmission provides the reference for subsequent delta position transmissions that may follow. A delta position transmission, or transmission of a numeric value representing the latitude and longitude coordinates of a subsequent position relative to the most recently transmitted absolute coordinates, is sent as long as the object remains within the most recently established geographic grid area. When the mobile object travels beyond the grid area, a new full position transmission is sent. In this manner, the use of capacity inefficient full position transmissions is reduced.
Exemplary methods and systems according to this invention do not require the use of tokens or other identification means representing constant or predefined absolute reference geographic coordinates in order to receive or transmit absolute or relative latitude and longitude coordinate data. Likewise, prior knowledge by the mobile object or the principal entity of one or more fixed or predefined geographic reference points, either preloaded or downloaded via a wireless communications systems, is not required for receipt or transmission of absolute or relative latitude and longitude coordinate data.